This invention relates to ultrasonic imaging of contrast agent and, more particularly, to an ultrasonic imaging method that enhances ultrasound returns from contrast agent by substantially reducing returns from tissue and blood.
Currently, there are several techniques for imaging contrast agents in the body, which enhance the sensitivity to contrast agent relative to body tissue and clutter. These techniques exploit the non-linear behavior of contrast agents.
U.S. Pat. No. 5,577,505 to Brock-Fisher et al., assigned to the same Assignee as this Application, measures ultrasound response under multiple excitation levels. It is known that the ultrasound responses from contrast agent increase somewhat exponentially with increases in applied ultrasound pressure. In contrast responses from tissue and blood are substantially linear with increases in ultrasound energy. In the Brock-Fisher et al. patent, ultrasound responses are gain corrected by an amount that corresponds to the difference in excitation levels and are then subtracted. Because of this subtraction, most of the linear response that is characteristic of tissue is removed and what remains is the non-linear response that results from contrast agent.
U.S. Pat. No. 5,632,277 to Chapman et al. enhances the ultrasound returns from contrast agent by altering the carrier phase by 180 degrees as between two successive transmit events. The echo signals are measured and combined so that the linear components cancel, leaving the nonlinear components for analysis.
U.S. Pat. No. 5,706,819 to Hwang et al. inverts the polarity of the succeeding transmit waveforms. The echo signals received from the succeeding transmit events are combined and a harmonic response is obtained that enables the nonlinear echoes from contrast agent to be detected.
U.S. Pat. No. 5,902,243 to Holley et al. modulates the phase of one or more components of the transmit waveform. Received echoes are summed to selectively cancel the harmonic or fundamental components. Such action can either enhance the fundamental echo frequencies or the harmonic frequencies.
European Patent Application EP 0913704 A2 discloses an imaging method which separates broadband linear and nonlinear echo signal components. The method maps echo signals in the time domain to Doppler shift frequencies in the frequency domain in a manner that depends upon the linearity of the echoes. The method analyzes phase shifts between successive echoes and the resulting Doppler spectrum is separated into even and odd harmonics. The resulting signals are separated into phase shift information and linear and nonlinear components to enable discrimination of motion artifacts.
In all of the above listed techniques, there is no differentiation between the nonlinear effects that result from tissue echoes versus the nonlinear effects of that result from contrast agent echoes. At moderate to high transmit pressures, significant nonlinear signals arising from the body fluids and tissues are detected and reduce the ability to differentiate contrast agents from surrounding tissues.
Accordingly, notwithstanding the multiple methods mentioned above for enhancing echo returns from contrast agent, there still exists a need for an improved method for enhancement of contrast agent response that enables a differentiation between nonlinear returns from contrast agent and tissue.
The invention suppresses nonlinear echo signals arising from tissue, while allowing the nonlinear signals from the contrast agent to pass essentially unaffected for further processing. This is made possible due to a fundamental difference in the nonlinear response of body tissues, as compared with contrast agents. More particularly, the nonlinear response of tissues can be characterized as a xe2x80x9czero-memoryxe2x80x9d effect, in that the response of a scatterer in the tissue is proportional to the instantaneous acoustic pressure at the scatterer. However, the response of contrast agents has been found to be affected by the acoustic pressure waveform over a considerable period of time. Thus, contrast agents in acoustic pressure fields do not behave as zero memory systems. In other words, subsequent echo responses from contrast agent do not exhibit such a proportionality, as they are affected by previous contrast agent responses.
The method of the invention enhances echo responses from contrast agent in relation to echo responses from tissue, where the echo responses from tissue exhibit a relationship that conforms to a polynomial model while the echo responses from the contrast agent do not conform thereto. The method causes transmission of plural ultrasound signals into a body including the contrast agent, each of the plural ultrasound signals transmitted with a predetermined transmit gain factor. The echo signals resulting from the plural ultrasound signals are received and each thereof is received with a predetermined receive gain factor. The transmit gain factor and receive gain factor for each corresponding transmitted ultrasound signal and echo signal are set to render the polynomial model equal to zero. The received echo signals are then combined in a manner to eliminate a fundamental and at least one harmonic component that conform to the polynomial model so as to leave signal components that do not conform (i.e., those that result from the contrast agent).